In recent years, algae (in particular, microalgae) which can produce a physiologically active substance such as a biofuel (hydrocarbon or bio-diesel) and astaxanthin have attracted attention. A technology has been studied in which such algae are cultured in large quantity and products thereof are used as substitute energy sources for oil or are used to produce drugs, cosmetics, foods, or the like.
A culture device for large-scale culture of algae or the like includes a culture device in which a water surface of a culture solution is exposed to the atmosphere (open system reactor), for example, a raceway type or open pond type culture device (culture pond) (for example, refer to Non-Patent Document 1). Plants, such as the algae, photosynthesize by using carbon dioxide (CO2) as a carbon source under a condition of light irradiation so as to proliferate or to produce products such as hydrocarbons. Therefore, in order to improve the production efficiency of the products such as the biofuel and the physiologically active substance, it is desirable to allow light to reach inside the culture pond and to sufficiently dissolve carbon dioxide into the culture solution through bubbling supply.
In the raceway type culture device, there is a trade-off relationship between allowing light to reach inside the culture pond and dissolving carbon dioxide into the culture solution using the bubbling supply or the like.
To be more specific, in the raceway type culture device, the light enters inside the culture pond only from the water surface. Thus, if the algae proliferate, the algae block the light themselves, whereby the reaching distance of the light may be shortened, and the efficiency of photosynthesis of the algae may be deteriorated. Therefore, the raceway type culture device is formed to be shallow to some degree (for example, approximately 30 cm) so that the light reaches a bottom surface of the culture pond even if the algae proliferate.
However, if a depth (vertical length) of the culture pond is shortened in view of the light reaching, even if the carbon dioxide is supplied into the culture solution using the bubbling supply or the like, the supplied carbon dioxide may be immediately released to the atmosphere from the water surface. Thus, it may not possible to ensure time for contact between the carbon dioxide and the culture solution, thereby causing insufficient dissolving of the carbon dioxide into the culture solution. In addition, a concentration of the carbon dioxide dissolved into the culture solution generally varies until the concentration reaches equilibrium with a concentration of the carbon dioxide in the atmosphere. Thus, even if the culture pond is supplied with the culture solution in which the carbon dioxide is dissolved in advance so as to have a high concentration (for example, approximately 2% to 3%), since the raceway type culture device has the water surface exposed to the atmosphere, the concentration of the carbon dioxide dissolved into the culture solution is decreased to the concentration of the carbon dioxide in the atmosphere (approximately 400 ppm) in a short period of time.
Therefore, a technology is disclosed which separately includes a culture device which dissolves high concentration carbon dioxide into an alga solution as the culture solution containing algae; and a culture pond which accommodates the alga solution having the dissolved high concentration carbon dioxide and delivered from the culture device and in which the photosynthesis of the algae is performed (for example, refer to Patent Document 1). In the technology disclosed in Patent Document 1, a box-shaped cover member (lid) composed of plate-shaped members made of vinyl resin or acrylic resin is installed on an outer edge of the culture pond, thereby covering an opening of the culture pond, and thus the carbon dioxide in the culture pond is prevented from being released to the atmosphere.